It is categorized as a rare ingredient, meaning that most apothecary merchants have a 21% chance of carrying 1-4 samples. In addition, it may be randomly found only in rare-type apothecary's satchels. Other possible locations for already-harvested samples include:

During this quest, you are asked to locate fine-cut void salts. These are just standard void salts (including same FormID), except given a different name. They appear under void salts during potion creation, and can be mixed with normal void salts.

Alternatively, remove all of your void salts (as they all have the same id) by using the command player.removeitem 3AD60 X (X being how many you have both fine-cut and regular combined). Then type player.additem 3AD60 X to get them back. You should now be able to use your void salts again.

Combining void salts with the fine-cut void salts will consume two void salts from your ordinary void salt pile, so long as you have two that can be consumed, allowing you to keep the fine-cut void salts. Doing this reveals all four effects of the void salts.